Introduce rule task.

[Masayuki Takagi]

Add rule task that represents generating a group of files from another in generalized manner. Groups of files are specified with file extensions.

For example, a rule that compiles C source files, having .c extension, to obtain object files, having .o extension, would be expressed as following.

(rule ".o" (".c") task
  (loop for source in (rule-task-sources task)
     do (sh "gcc -c ${source}")))